The field of the invention relates to air/fuel ratio control for motor vehicles having a fuel vapor recovery system coupled between the fuel supply system and the air/fuel intake of an internal combustion engine.
Efficient operation of internal combustion engines requires the engine's air/fuel ratio be maintained within an operating window of the catalytic converter. For a typical three-way catalytic converter (NO.sub.X, CO, and HC), the desired air/fuel ratio is referred to as stoichiometry which is typically 14.7 lbs. air/lb. fuel. Engine operation at the desired air/fuel ratio is approached by an air/fuel ratio feedback control system responsive to an exhaust gas oxygen sensor. More specifically, a fuel charge is first determined for open loop operation by dividing a measurement of inducted airflow by the desired air/fuel ratio (such as 14.7). This open loop charge is then trimmed by a feedback correction factor responsive to the exhaust gas oxygen sensor. In this manner, steady-state engine operation is maintained near the desired air/fuel ratio.
Air/fuel ratio control has been complicated by the addition of fuel vapor recovery systems. To reduce emissions of gasoline vapors into the atmosphere, as required by government emission standards, fuel vapor recovery systems are commonly utilized. These systems store excess fuel vapors emitted from the fuel tank in a canister having activated charcoal or other hydrocarbon absorbing material. To replenish the canisters storage capacity, air is periodically purged through the canister, absorbing stored hydrocarbons, and the mixture of vapors and purged air inducted into the engine. Concurrently, vapors are inducted directly from the fuel tank into the engine.
A problem with the above described approach to air/fuel ratio control is that induction of rich fuel vapors may exceed the feedback system's range of authority resulting in undesired engine air/fuel operation. Another problem is that any perturbation in inducted airflow, such as caused by sudden changes in throttle position, results in an air/fuel transient due to the feedback systems response time.
U.S. Pat. No. 4,715,340 has attempted to solve the above problems. A combined air/fuel ratio feedback control system and vapor purge system is disclosed wherein the rate of purged vapor flow is made proportional to the rate of inducted airflow. Allegedly, any change in inducted airflow will then be accompanied by a corresponding change in purged vapor flow such that the over all air/fuel ratio is not significantly perturbed during sudden changes in throttle position.
U.S. Pat. No. 4,641,623 addresses another of the above described problems. To reduce air/fuel transients which may be caused by the onset of fuel vapor purge, the '623 patent discloses gradually ramping on purge flow such that the feedback system may gradually track the inducted change in an air/fuel mixture.
Kortge et al U.S. Pat. No. 4,741,318 addresses the above described problem of purge fuel vapors exceeding the feedback system's range of authority. Kortge et al discloses a feedback control system wherein the output of an exhaust gas oxygen sensor is integrated to provide a correction factor for injected liquid fuel. During fuel vapor purging, the rate of purge flow is increased until such integrated value exceeds a predetermined value associated with the limit of the feedback system's range of authority. When this value is reached, further increases in the rate of purge flow are either inhibited or the rate of purge flow is decreased. Accordingly, the rate of purge flow is continuously adjusted such that induction of purged fuel vapors does not exceed the feedback system's range of authority.
The inventors herein have recognized numerous disadvantages with the above described prior art approaches. For example, the '318 patent and the '340 patent teach limiting the rate of purge flow such that the feedback system's range of authority is not exceeded. A disadvantage of these approaches is that fuel vapors may be generated in the fuel system at a greater rate than they are purged into the engine. Accordingly, the vapor storage canister may become saturated and excess fuel vapors emitted directly into the atmosphere.